(1) Field of the Invention
The present invention relates to a voltage regulator circuit for providing a constant output voltage at its output terminal particularly when a change to the input voltage occurs within a predetermined range of the voltage fluxiation.
(2) Description of The Prior Art
As is well known, voltage regulators are used to meet the requirement for a constant output voltage by a power supply regardless of an input voltage or load variations. The voltage regulator is usually in the form of a device connected with the output of a power supply to maintain the output voltage at a rated value. The regulator is usually required to act automatically to compensate for changes that occur in the operation of the circuit.
A conventional voltage regulator circuit is illustrated schematically in FIGS. 1A and 1B and includes a plurality of N-type metal-oxide-transistors(MOS) connected in series each having its gate connected to a drain between a supply Vcc and ground such that an output voltage Vo is obtained at a node na of NMOS transistor. According to this voltage regulator circuit, because NMOS transistors are in diode configuration, the output voltage Vo of the circuit is increased proportionally as the supply voltage Vcc is increased as shown in a graph of the relationship between a supply voltage and an output voltage in the circuit of FIG. 1B.
Another conventional voltage regulator circuit is illustrated schematically in FIG. 2A and is an improvement to the conventional voltage regulator circuit shown in FIG. 1A. In a manner similar to voltage regulator of FIG. 1A, NMOS transistors are connected in series and each has its gate connected to its drain but the series connection is between a voltage supply Vcc and node na. A variable resistor is effectively formed by NMOS transistors having a supply voltage Vcc supplied to their gates while serially connected between node and ground. This use of NMOS transistors as a variable resistor enables operation of the voltage regulators such that when the supply voltage Vcc increases, there is a reduction to the resistance by the NMOS transistors serially connected between Vo and Vcc. The output voltage Vo can decrease in proportion to an increase of the supply voltage Vcc. In the voltage regulator circuit of FIG. 2A, the output voltage characteristic is improved as compared to the conventional voltage regulator circuit of FIG. 1A, but the output voltage Vo still increases in response to an increasing supply voltage Vcc as depicted by the graph line of FIG. 2B.
Accordingly, an object of the present invention is to provide an improved voltage regulator circuit for providing a constant output voltage, even when there are changes to the input supply voltage Vcc.